Experimental investigation of self-induced thermocapillary convection for an evaporating meniscus in capillary tubes using micro–particle image velocimetry

Abstract

The present paper reports an experimental investigation of the self-induced liquid convection for an evaporating meniscus in small capillary tubes. The strong evaporative cooling at the triple contact line leads to a variation in temperature along the liquid–vapor interface, which generates a gradient of surface tension that in turn drives the observed convection. Ethanol and methanol in three tube sizes (600, 900, and 1630 μm) were investigated in this study. The flow pattern in the liquid phase has been characterized using a micro–particle image velocimetry (PIV) technique with a vector spatial resolution of 640 nm. Thermocapillary Marangoni convection is observed in horizontal diametrical sections of the horizontally oriented capillary tubes as two contrarotating vortices of similar strength, whereas in vertical diametrical sections a single clockwise vortex is mostly present. This distortion of the flow pattern could be attributed to gravity. The distortion and loss of symmetry in the vertical section is found to exhibit an oscillatory behavior. The convection (represented by the vorticity) is found to be stronger for more volatile liquids and smaller tube sizes. The vorticity normalized with the convective time scale is found to be higher for the less volatile liquid and to increase with the tube radius. Therefore, a further correction of the normalized vorticity using a dimensionless liquid saturated vapor pressure leads to a parameter that is found independent of the tube size and the liquid properties, suggesting that the phenomena described here are universal and dictated by the local conditions near the triple line.